1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) optical communication system.
2. Description of the Related Art
Recently, in order to greatly increase the transmission capacity of a communication line, a WDM optical communication system has been actively studied and developed. In the WDM optical communication system, each channel is allocated to a light of a different wavelength, the light of each wavelength is modulated, and the wavelengths are multiplexed to be transmitted. Transmission characteristics of the communication system are measured by the SN ratio (OSNR) of an optical signal. Even if the levels of the optical signals of respective wavelengths are matched with each other to be transmitted on a transmission side, they become different on a reception side, due to the gain characteristics of an optical relay or the like, which is provided in a transmission path. That is, since the OSNR of the optical signal of a specific wavelength deteriorates among optical signals which are multiplexed many times, the transmission characteristics of a system are evaluated by the worst OSNR although the optical signal of a wavelength having a good OSNR is present. Therefore, in the WDM optical communication system, when an optical signal is received at a receiving station, all the OSNRs of the optical signals of wavelengths are required to be similarly good values. A pre-emphasis technology is adopted for adjusting the OSNRs of optical signals of respective wavelengths, which are received at the receiving station side, to be almost at the same level.
FIGS. 1A and 1B are diagrams explaining a pre-emphasis technology.
FIG. 1A shows a case where a pre-emphasis process is not performed on a transmission side. That is, the power levels of optical signals of respective wavelengths are matched to be transmitted. The OSNRs of the thus-transmitted optical signals deteriorate because of the gain characteristics of an optical ampliifier (relay) which is provided in a transmission path. That is, when the optical signal is amplified by the optical amplifier, an ASE (Amplified Spontaneous Emission) is added to the optical signal as noise, and the gain characteristics of the optical amplifier become different depending on the wavelength position where optical signals of respective wavelengths are located, as shown in the middle of FIG. 1A, so that a level difference between optical signals is generated. Therefore optical signals of different levels with noise added will be received for each wavelength, on a reception side. The right side of FIG. 1A shows such a condition. Accordingly, a deviation is generated in the OSNRs of the optical signals.
FIG. 1B is a diagram explaining a case where pre-emphasis is performed on a transmission side.
When a pre-emphasis process is performed, the following operations are performed, and the optical signals are transmitted. The power of an optical signal of a wavelength whose OSNR deteriorates when it is received on a reception side, is preliminarily increased. By contrast, the power of the optical signal of a wavelength whose OSNR has not deteriorated so much when it is received, is relatively decreased. When such a wavelength-multiplexed optical signal is transmitted, the optical signal of a wavelength having a relatively low power at a transmission time, is greatly amplified in a transmission path. However, the optical signal of a wavelength of a relatively high power at a transmission time is only slightly amplified. Accordingly, optical signals of wavelengths whose power levels are relatively equal can be received on a reception side. The right side of FIG. 1B shows such a condition. As shown in the middle of FIG. 1B, gain characteristics are strong around the center, and they become smaller further from the center. Therefore, when a pre-emphasis process is performed, the power level of an optical signal of a wavelength located at the periphery of a gain band is adjusted to be increased.
Really, since there is scarcely any case where the gain characteristics of all of the transmission paths are preliminarily recognized, it is adjusted how to perform a pre-emphasis process, taking into account the state of an optical signal on a reception side. Since noise together with an optical signal are received on a reception side, a state where the OSNRs of optical signals of respective wavelengths are equal is more preferable to a transmission characteristic than a state where the power levels of optical signals of respective wavelengths are equal. Therefore, the OSNRs between wavelengths are adjusted to be matched with each other when optical signals are received in a pre-emphasis process.
Recently, in order to more flexibly correspond an optical communication network to users"" requests, a configuration of an optical add/drop multiplexing (OADM) system has been required. In this system, the optical signal of a specified wavelength is branched into a transmission path for connecting two trunk stations which are terminal stations of the trunk transmission path, so that the thus-branched optical signals are transmitted to another terminal station (branch station), or an optical signal of the specified wavelength transmitted from the branch station is inserted to the trunk transmission path. In the OADM system, a branching unit (BU) for branching and inserting a specified wavelength into/to a transmission path is provided. The branching unit to be used in an OADM system is called an OADM BU.
In a system where an OADM BU is not included such as a linear network for connecting two terminal stations which face to each other, a deviation of OSNRs for the optical signals of respective wavelengths can be controlled only by performing a strong pre-emphasis process to the optical signals of lower OSNRs of the opposite station, and by performing a weak pre-emphasis process to those of high OSNRs. Therefore, a person can easily perform such a pre-emphasis process by hand.
In a system including the OADM BU system, however, since a plurality of stations determine how great a pre-emphasis process is performed, very complicated procedures are required so that time and labor are also required when a person performs such a pre-emphasis process by hand. Therefore, the deviation of OSNRs cannot be controlled because of deterioration caused by insertion of a cable, deterioration with time, and the failure of a transmission path, even if the amount of a pre-emphasis process to be performed for each wavelength is initially set. Accordingly, this will be a big problem especially in a long distance transmission system.
Even if each receiving station individually performs a pre-emphasis process in order to control the deviation in the receiving station, pre-emphasis processes can hardly converge since optical signals affect each other in a transmission path. Conventionally, there is no method for performing a pre-emphasis process in short time. Therefore, a method for realizing this process has been required.
The subject of the present invention is to provide a system for automatically perform a pre-emphasis process in short time in a WDM optical communication network where a plurality of terminal stations are connected.
The present invention comprises (1) a first terminal station having a control means for terminating a trunk transmission path in a WDM optical communication system where a plurality of terminal stations are connected by a network, and for controlling a pre-emphasis process which is performed at a terminal station for transmitting optical signals among terminal stations included in the WDM optical communication system, (2) a branching and inserting means for branching and inserting the optical signal of a predetermined wavelength among wavelength-multiplexed optical signals which propagate through the trunk transmission path, and (3) a second terminal station connected to the branching and inserting means, for receiving the optical signal of the predetermined wavelength from the trunk transmission path, and for transmitting the optical signal of the predetermined wavelength to be inserted to the trunk transmission path. The present invention is characterized in that when a pre-emphasis process is performed for a wavelength-multiplexed optical signal to be transmitted in such a direction that the first terminal station becomes a reception terminal station in the wavelength division multiplexing optical system, the control means obtains the spectrum information of an optical signal received by the first terminal station, causes the second terminal station to transmit the spectrum information of an optical signal to be received by the first terminal station, obtains the spectrum information received from the second terminal station, and instructs a terminal station which transmits an optical signal about how great a pre-emphasis process should be performed, using the spectrum information which is obtained from the first and second terminal stations.
In a wavelength division multiplexing system having a network configuration where a plurality of branch transmission paths are provided at a trunk transmission path, and one or more terminal stations are connected to the branch transmission path, a terminal station of the present invention is one connected to the trunk transmission path. This terminal station of the present invention is characterized in that it obtains, from a terminal station which receives a wavelength division multiplexed optical signal, spectrum information of the thus-received optical signal by the terminal station, and it comprises a control means for instructing how great a pre-emphasis process should be performed for a terminal station for transmitting an optical signal according to the spectrum information.
According to the present invention, since pre-emphasis processes of terminal stations which are connected to a system are simultaneously controlled by a control means which is provided at a single terminal station, no confusion occurs when controlling the pre-emphasis processes. Therefore, the processes can be terminated in short time.
Since a pre-emphasis process is automatically performed by the control means, it is not necessary for a person to perform complicated operations spending relatively a long time. Accordingly, the present invention can obtain a good cost performance.